The easiest way to put black borders on a print is to make a mask smaller. Now, instead of trying to put it in the center, do this two step process: Put the mask in the upper left corner right at the print edge, and expose white light. Then slide the mask to the bottom right corner (diagonal), and expose again. Real easy to do and get the borders even.

The easiest way to get black border with a white border outside of it is to do exactly as above, but with the paper held by a four-blade easel. The blades form the outer mask.

Of course, you need one mask for each print size, but you probably only have 8x10 or 5x7.

You could always overmount with black mat and not touch the print.

P.S. Just a comment on Thomas's mentor: What are we, lawyers?